Poly(alkylene terephthalate) has excellent physical properties such as abrasion resistance, durability, heat stability, etc., and thus has been used as a material for making fiber, film and molding products. Poly(ethylene terephthalate) (hereinafter, referred to as ‘PET’), poly(butylene terephthalate) (hereinafter, referred to as ‘PBT’), and polycyclohexylenedimethylene terephthalate (poly(1,4-cyclohexylenedimethylene terephthalate), hereinafter, referred to as ‘PCT’) are commercial polyalkylene terephthalate. Among them, the most widely used commercial material is PET that is mainly used for fibers, bottles, or the like.
In spite of its excellent physical properties, PET needs a nucleating agent and a crystallization accelerator as a processing aid when used as engineering plastics that are required to have high crystallinity, and the production rate becomes low or the mold temperature must be controlled to maintain a high crystallization rate during an injection molding process, because it has a relatively low crystallization rate.
Meanwhile, since PBT has a higher crystallization rate than PET, it overcomes the above mentioned problems in physical properties of PET used in engineering plastics, namely, the low crystallization rate, and thus has been widely used in engineering plastics. However, PBT has lower heat deflection temperature than PET, and thus its use in applications requiring high heat resistance has been limited, despite its excellent moldability compared to PET.
Meanwhile, PCT has attracted much attention as a new material capable of overcoming the above described problems of polyester materials, that is, the moldability problem due to slow crystallization rate and the limited applications due to low heat deflection temperature.
Such PCT is a crystalline polyester prepared by esterification or ester exchange reaction and polycondensation reaction of terephthalic acid (hereinafter, referred to as TPA) or dimethyl terephthalate (hereinafter, referred to as DMT) and 1,4-cyclohexanedimethanol (hereinafter, referred to as CHDM), and has a very high melting point (Tm) and a very fast crystallization rate. Since the first development in 1960, PCT have been mainly applied to carpets owing to a soft touch of the PCT fiber. However, the use of PCT has been gradually declining with the emergence of polyamide. Since PCT compound formulations were developed in the engineering plastic fields in the 1980s, they have been applied only to the connectors and heat resistant parts in electrical, electronic and automobile fields requiring high heat-resistance.
PCT has excellent heat resistance, chemical resistance, moisture resistance and flowability, compared to the widely used polyesters, PET and PBT. In particular, PCT has a heat deflection temperature of 245 to 260° C. and a continuous-use temperature of 130 to 150° C. Therefore, of the commercial non-wholly aromatic polyesters, excluding liquid crystalline polyesters, PCT is the only engineering plastic belonging to super engineering plastics as alternatives to metals, such as polyamide, polyphenylene sulfide, and liquid crystalline polymer. Particularly, PCT has very excellent color stability and remarkably low water absorption rate, compared to other polymer resins such as polyamide or the like, and therefore, it can be usefully applied to electronic materials that are produced by high temperature surface mount technology or housings or reflectors of LED (Light Emitting Diode) that are continuously exposed to heat and light during use of the product.
U.S. Pat. No. 5,106,944 discloses a process for preparing PCT using DMT and CHDM as main materials and titanium alkoxide and alkaline earth metal salts as catalysts, and U.S. Pat. No. 5,124,388 discloses a technology for improving colors of PCT copolyester and PCT copolyester/polycarbonate blend by using hindered phenolic stabilizers. In these patents, however, germanium compounds were not used as a catalyst.
Further, U.S. Pat. No. 5,596,068 discloses a polyester resin for producing thick-walled bottles having high transparency and a neutral color by using an antimony compound, a germanium compound, and a phosphorus compound. However, this patent is characterized by including CHDM of 0.5˜15% by weight and the resin has a great difference with PCT resins. In particular, there is no mention of the remarkable improvement in its color and light reflectance by use of the germanium catalyst.
U.S. Pat. No. 4,972,015 discloses a thin-walled, thermoformed, heat-set article, which is prepared by using PCT and PCT copolyester having an intrinsic viscosity of 0.7˜1.1, and U.S. Pat. No. 5,242,967 discloses a method for improving crystallization characteristics of PCT by addition of aliphatic polyester. Further, U.S. Pat. No. 4,859,732 discloses a PCT compound formulation that is added with linear alcohols and glass reinforcing fibers to improve crystallization characteristics and strength of PCT.
However, the conventional technologies proposed only the composition for improving crystallization characteristics and color in the compound step, and there have been no reports on a preparation method capable of basically improving color stability and heat stability of PCT in a PCT polymerization step.